The present invention relates to a power amplifier; and, more particularly, to a power amplifier including a bias current control circuit capable of effectively reducing a quiescent current of the amplifier to improve the power added efficiency (PAE) thereof.
As is well known, a power amplifier is one of major power consuming components of a cell phone. FIGS. 1A and 1B illustrate a typical prior art power amplifier module for use in a conventional CDMA cell phone.
The power amplifier shown in FIG. 1A includes a bias circuit 101 in addition to an amplifying circuit. The amplifying circuit includes an amplifying transistor Q1 having an emitter grounded; an inductor L, one end thereof being supplied with Vcc and the other end thereof being connected to a collector of Q1; an output capacitor Co disposed between the collector of Q1 and an RF_OUT terminal; and an input capacitor Ci coupled between an RF_IN terminal and a base of Q1.
The bias circuit 101 is of a current mirror structure including a bias transistor Q2, a collector thereof being supplied with Vref; a bias transistor Q3, an emitter thereof being grounded; a resistor R1, one end thereof being supplied with Vref and the other end thereof being connected to a base of Q2 and a collector of Q3; a resistor R2, one end thereof being connected to the base of Q3 and the other end thereof being coupled to an emitter of Q2; and a resistor R3, one end thereof being coupled to a node between Ci and the base of Q1 and the other end thereof being coupled to the emitter of Q2.
The power amplifier shown in FIG. 1B includes a bias circuit 102 in addition to an amplifying circuit identical to that shown in FIG. 1A. The bias circuit 102 is also of a current mirror structure including a bias transistor Qbias, a collector thereof being supplied with Vref; an emitter-base diode (i.e., a bipolar transistor with short-circuited collector and base) D1, an anode thereof being connected to a base of Qbias; an additional emitter-base diode D2, an anode thereof being connected to a cathode of D1 and a cathode thereof being grounded; and a resistor Rbias, one end thereof being supplied with Vref and the other end thereof being connected to the anode of D1.
Referring to FIGS. 1A and 1B, once Vref is set to have a certain value, IB (a bias current of Q1, i.e., a DC component of a base current of Q1) is fixed regardless of an output power. That is to say, the bias circuit 101 or 102 supplies a constant bias current regardless of the output power, which in turn gives rise to a constant quiescent current Ic (i.e., a DC component of a collector current of Q1), IC being an operation current of Q1.
A maximum output power is one of most important performance figures for such power amplifiers. However, such power amplifiers are rarely in operation at the maximum output power of, e.g., 28 dBm, but mostly operate at low output power levels less than, e.g., 16 dBm. Therefore, it is required to control operation currents to be reduced at the low output power levels so that we can improve the PAE (power added efficiency) of CDMA power amplifiers.
Various research efforts have been made for a PAE improvement by controlling a bias with an aid of an additional circuitry. For example, an ABC (an automatic bias control) system was proposed to decrease the bias current by way of adjusting Vref at the low output power levels (see, e.g., T. Sato et al., xe2x80x9cIntelligent RF power module using automatic bias control (ABC) system for PCS CDMA applicationsxe2x80x9d, IEEE MTT-S Int. Microwave Simp. Dig., 1998, pp.201-204). Since, however, the ABC system requires a separate ABC-chip in addition to an MMIC (monolithic microwave integrated circuit) incorporating therein a power amplifier circuitry, the size of the power amplifier module increases.
For other examples, a dynamic supply voltage (VCC) and current adjustment based on envelope detection were proposed, where additional components such as a dc-dc converter, an envelope detector and a coupler are required (see, e.g., M. Ranjan et al., xe2x80x9cMicrowave power amplifiers with digitally-controlled power supply voltage for high efficiency and high linearityxe2x80x9d, IEEE MTT-S Int. Microwave Simp. Dig., 2000, pp.493-496; and Yang Kyounghoon et al., xe2x80x9cHigh efficiency class-A power amplifiers with a dual-bias-control schemexe2x80x9d, IEEE Trans. Microwave Theory Tech., vol.47, pp.1426-1432, August 1999). In these schemes, however, it is difficult to integrate the additional components (i.e., a dc-dc converter, an envelope detector and a coupler) in an MMIC together with power amplifiers because of the size or complexity of those components.
As described above, prior art schemes for controlling the bias of a CDMA power amplifier have drawbacks due to additional elements that substantially increase a chip area, and power consumption.
It is, therefore, an object of the present invention to provide a power amplifier module including a bias current control circuit that scarcely increases a chip area and power consumption.
In accordance with the present invention, there is provided a power amplifier for use in a mobile handset including: an amplifying transistor for generating an output of the mobile handset; a bias circuit having a bias transistor, the bias circuit providing a bias current to bias the amplifying transistor; and a bias current control circuit, responsive to a control signal, for adjusting the bias current to control an operation current of the amplifying transistor, wherein the control signal is determined by a power level of the output of the mobile handset.